Cellulose is a carbohydrate consisting of a long chain of glucose units, all β-linked through the 1′-4 positions. Native plant cellulose molecules may have upwards of 2200 anhydroglucose units. The number of units is normally referred to as degree of polymerization (D.P.). Some loss of D.P. inevitably occurs during purification. A D.P. approaching 2000 is usually found only in purified cotton linters. Wood derived celluloses rarely exceed a D.P. of about 1700. The structure of cellulose can be represented as follows: 
Chemical derivatives of cellulose have been commercially important for almost a century and a half. Nitrocellulose plasticized with camphor was the first synthetic plastic and has been in use since 1868. A number of cellulose ether and ester derivatives are presently commercially available and find wide use in many fields of commerce. Virtually all cellulose derivatives take advantage of the reactivity of the three available hydroxyl groups (i.e., C2, C3, and C6). Substitution at these groups can vary from very low, about 0.01, to a maximum of 3. Among important cellulose derivatives are cellulose acetate, used in fibers and transparent films; nitrocellulose, widely used in lacquers and gunpowder; ethyl cellulose, widely used in impact resistant tool handles; methyl cellulose, hydroxyethyl, hydroxypropyl, and sodium carboxymethyl cellulose, water soluble ethers widely used in detergents, as thickeners in foodstuffs, and in papermaking. Cellulose itself has been modified for various purposes. Cellulose fibers are naturally anionic in nature, as are many papermaking additives. A cationic cellulose is described in U.S. Pat. No. 4,505,775, issued to Harding et al. This cellulose has greater affinity for anionic papermaking additives such as fillers and pigments and is particularly receptive to acid and anionic dyes. U.S. Pat. No. 5,667,637, issued to Jewell et al., describes a low degree of substitution (D.S.) carboxyethyl cellulose which, along with a cationic resin, improves the wet to dry tensile and burst ratios when used as a papermaking additive. U.S. Pat. No. 5,755,828, issued to Westland, describes a method for increasing the strength of articles made from crosslinked cellulose fibers having free carboxylic acid groups obtained by covalently coupling a polycarboxylic acid to the fibers.
For some purposes, cellulose has been oxidized to make it more anionic to improve compatibility with cationic papermaking additives and dyes. Various oxidation treatments have been used. Among these are nitrogen dioxide and periodate oxidation coupled with resin treatment of cotton fabrics for improvement in crease recovery as suggested by Shet, R. T. and A. M. Nabani, Textile Research Journal, November 1981: 740–744. Earlier work by Datye, K. V. and G. M. Nabar, Textile Research Journal, July 1963: 500–510, describes oxidation by metaperiodates and dichromic acid followed by treatment with chlorous acid for 72 hours or 0.05 M sodium borohydride for 24 hours. Copper number was greatly reduced by borohydride treatment and less so by chlorous acid. Carboxyl content was slightly reduced by borohydride and significantly increased by chlorous acid. The products were subsequently reacted with formaldehyde. Southern pine kraft springwood and summer wood fibers were oxidized with potassium dichromate in oxalic acid. Luner, P., et al., Tappi 50(3):117–120 (1967). Handsheets made with the fibers showed improved wet strength believed to be due to aldehyde groups. Pulps have also been oxidized with chlorite or reduced with sodium borohydride. Luner, P., et al., Tappi 50(5):227–230, 1967. Handsheets made from pulps treated with the reducing agent showed improved sheet properties over those not so treated. Young, R. A., Wood and Fiber 10(2):112–119, 1978 describes oxidation primarily by dichromate in oxalic acid to introduce aldehyde groups in sulfite pulps for wet strength improvement in papers. Shenai, V. A. and A. S. Narkhede, Textile Dyer and Primer, May 20, 1987: 17–22 describes the accelerated reaction of hypochlorite oxidation of cotton yarns in the presence of physically deposited cobalt sulfide. The authors note that partial oxidation has been studied for the past hundred years in conjunction with efforts to prevent degradation during bleaching. They also discuss in some detail the use of 0.1 M sodium borohydride as a reducing agent following oxidation. The treatment was described as a useful method of characterizing the types of reducing groups as well as acidic groups formed during oxidation. The borohydride treatment noticeably reduced copper number of the oxidized cellulose. Copper number gives an estimate of the reducing groups such as aldehydes present on the cellulose. Borohydride treatment also reduced alkali solubility of the oxidized product, but this may have been related to an approximate 40% reduction in carboxyl content of the samples. Andersson, R., et al. in Carbohydrate Research 206: 340–346 (1990) describes oxidation of cellulose with sodium nitrite in orthophosphoric acid and describe nuclear magnetic resonance elucidation of the reaction products.
Davis, N. J., and S. L. Flitsch, Tetrahedron Letters 34(7): 1181–1184 (1993) describe the use and reaction mechanism of 2,2,6,6-tetramethylpiperidinyloxy free radical (TEMPO) with sodium hypochlorite to achieve selective oxidation of primary hydroxyl groups of monosaccharides. Following the Davis et al. paper this route to carboxylation then began to be more widely explored. de Nooy, A. E. J., et al., Receuil des Travaux Chimiques des Pays-Bas 113: 165–166 (1994) reports similar results using TEMPO and hypobromite for oxidation of primary alcohol groups in potato starch and inulin. The following year, these same authors in Carbohydrate Research 269:89–98 (1995) report highly selective oxidation of primary alcohol groups in water soluble glucans using TEMPO and a hypochlorite/bromide oxidant.
WO 95/07303 (Besemer et al.) describes a method of oxidizing water soluble carbohydrates having a primary alcohol group, using TEMPO with sodium hypochlorite and sodium bromide. Cellulose is mentioned in passing in the background although the examples are principally limited to starches. The method is said to selectively oxidize the primary alcohol at C-6 to carboxylic acid group. None of the products studied were fibrous in nature.
WO 99/23117 (Viikari et al.) describes oxidation using TEMPO in combination with the enzyme laccase or other enzymes along with air or oxygen as the effective oxidizing agents of cellulose fibers, including kraft pine pulps.
A year following the above noted Besemer publication, the same authors, in Cellulose Derivatives, Heinze, T. J. and W. G. Glasser, eds., Ch. 5, pp. 73–82 (1996), describe methods for selective oxidation of cellulose to 2,3-dicarboxy cellulose and 6-carboxy cellulose using various oxidants. Among the oxidants used were a periodate/chlorite/hydrogen peroxide system, oxidation in phosphoric acid with sodium nitrate/nitrite, and with TEMPO and a hypochlorite/bromide primary oxidant. Results with the TEMPO system were poorly reproduced and equivocal. In the case of TEMPO oxidation of cellulose, little or none would have been expected to go into solution. The homogeneous solution of cellulose in phosphoric acid used for the sodium nitrate/sodium nitrite oxidation was later treated with sodium borohydride to remove any carbonyl function present.
Chang, P. S. and J. F. Robyt, Journal of Carbohydrate Chemistry 15(7):819–830 (1996), describe oxidation of ten polysaccharides including α-cellulose at 0 and 25° C. using TEMPO with sodium hypochlorite and sodium bromide. Ethanol addition was used to quench the oxidation reaction. The resulting oxidized α-cellulose had a water solubility of 9.4%. The authors did not further describe the nature of the α-cellulose. It is presumed to have been a so-called dissolving pulp or cotton linter cellulose. Barzyk, D., et al., in Transactions of the 11th Fundamental Research Symposium, Vol. 2, 893–907 (1997), note that carboxyl groups on cellulose fibers increase swelling and impact flexibility, bonded area and strength. They designed experiments to increase surface carboxylation of fibers. However, they ruled out oxidation to avoid fiber degradation and chose to form carboxymethyl cellulose in an isopropanol/methanol system.
Isogai, A. and Y. Kato, in Cellulose 5:153–164, 1998 describe treatment of several native, mercerized, and regenerated celluloses with TEMPO to obtain water soluble and insoluble polyglucuronic acids. They note that the water soluble products had almost 100% carboxyl substitution at the C-6 site. They further note that oxidation proceeds heterogeneously at the more accessible regions on solid cellulose.
Kitaoka, T., A. Isogai, and F. Onabe, in Nordic Pulp and Paper Research Journal 14(4):279–284, 1999, describe the treatment of bleached hardwood kraft pulp using TEMPO oxidation. Increasing amounts of carboxyl content gave some improvement in dry tensile index, Young's modulus, and brightness, with decreases in elongation at breaking point and opacity. Other strength properties were unaffected. Retention of PAE-type wet strength resins was somewhat increased. The products described did not have any stabilization treatment after the TEMPO oxidation.
U.S. Pat. No. 6,379,494 describes a method for making stable carboxylated cellulose fibers using a nitroxide-catalyzed process. In the method, cellulose is first oxidized by nitroxide catalyst to provide carboxylated as well as aldehyde and ketone substituted cellulose. The oxidized cellulose is then stabilized by reduction of the aldehyde and ketone substituents to provide the carboxylated fiber product. Nitroxide-catalyzed cellulose oxidation occurs predominately at the primary hydroxyl group on C-6 of the anhydroglucose moiety. In contrast to some of the other routes to oxidized cellulose, only very minor oxidation occurs at the secondary hydroxyl groups at C-2 and C-3.
In nitroxide oxidation of cellulose, primary alcohol oxidation at C-6 proceeds through an intermediate aldehyde stage. In the process, the nitroxide is not irreversibly consumed in the reaction, but is continuously regenerated by a secondary oxidant (e.g., hypohalite) into the nitrosonium (or oxyammonium or oxammonium) ion, which is the actual oxidant. In the oxidation, the nitrosonium ion is reduced to the hydroxylamine, which can be re-oxidized to the nitroxide. Thus, in the method, it is the secondary oxidant (e.g., hypohalite) that is consumed. The nitroxide may be reclaimed or recycled from the aqueous system.
The resulting oxidized cellulose product is an equilibrium mixture including carboxyl and aldehyde substitution. Aldehyde substituents on cellulose are known to cause degeneration over time and under certain environmental conditions. In addition, minor quantities of ketone may be formed at C-2 and C-3 of the anhydroglucose units and these will also lead to degradation. Marked degree of polymerization loss, fiber strength loss, crosslinking, and yellowing are among the consequent problems. Thus, to prepare a stabilized carboxylated product, aldehyde and ketone substituents formed in the oxidation step are reduced to hydroxyl groups, or aldehyde substituents are oxidized to a carboxyl group in a stabilization step.
In addition to TEMPO, other nitroxide derivatives for making carboxylated cellulose fibers have been described. See, for example, U.S. Pat. No. 6,379,494 and WO 01/29309, Methods for Making Carboxylated Cellulose Fibers and Products of the Method.
A method of preparation of carboxylic acids or their salts by oxidation of primary alcohols using hindered N-chloro hindered cyclic amines and hypochlorite, in aqueous solutions or in mixed solvent systems containing ethyleneglycol dimethyl ether, diethyleneglycol dimethyl ether, triethyleneglycol dimethyl ether, toluene, acetonitrile, ethylacetate, t-butanol and other solvents is described in JP10130195, “Manufacturing Method of Carboxylic Acid and Its Salts”. Other oxidants described include chlorine, hypobromite, bromite, trichloro isocyanuric acid, tribromo isocyanuric acid, or combinations.
Despite the advances made in the development of methods for making carboxylated cellulose pulps including catalytic oxidation systems, there remains a need for improved methods and catalysts for making carboxylated cellulose pulp. The present invention seeks to fulfill these needs and provides further related advantages.